If present levels of energy consumption per capita and lifestyles have to be maintained, the move towards a low-carbon energy society will need more renewable energy sources than previously believed. This was one of the key conclusions of a study that was recently reported in Nature Energy by Jeroen van den Bergh and Lewis King of the Institute of Science and Environmental Technology of the Universitat Autònoma de Barcelona (ICTA-UAB).

Post the Paris Agreement, a number of global energy transition scenarios were presented. Although these scenarios are likely to be studied in terms of gross energy, the study authors believe that energy requirements should be calculated by differentiating between net energy (gross energy minus the energy used to generate it) and gross (total energy produced). The concept of 'Energy Return on Investment' (EROI) is relevant in this context.

It represents the amount of useful energy produced for each unit of energy input during the process of acquiring that energy. If the EROI of an energy source is lower, more energy input will be needed to generate a given energy output. As a result, less net energy is available for consumption. Researchers believe that hydroelectricity and coal have high EROIs, while wind and solar power have medium to low EROIs, and oil, gas, and nuclear energy are characterized by medium EROIs.

A society's welfare, economy, and lifestyle is largely dependent on the EROI level. As soon as the requirements for basic consumption ("essentials"), for example, water and food are fulfilled, low-EROI economies would have less than 50% of the net energy of high-EROI economies available for production and consumption of all "non-essential" services and goods. This would not only have major implications for lifestyles, but would also restrict the ability to invest energy to realize future economic growth.

Therefore, in the face of an imminent scenario based on renewable energy sources (with low-EROI rates), the scientists believe that if significant investments are not made in energy efficiency, the net energy per capita will probably reduce in the coming days between 24% and 31% from 2014 levels.

"To maintain net energy per capita at current levels, renewable energy sources would have to grow at a rate two to three times that of current projections", said Lewis King.

The results also denote a prioritization in phasing out fossil fuels, for example, coal followed by oil and gas. A carbon price can be implemented to achieve this and would also discourage coal use more than oil, and oil use more than gas.

In order to enhance lifestyles, a low-EROI society has three options: enhance end use energy efficiency in production and consumption, boost gross energy production, or enhance the average EROI significantly through technological advancements and investment in higher-EROI energy sources.

The challenge of a rapid transition to low-carbon energy is therefore twofold: staying within the carbon budget associated with accepted climate change targets (2 ºC warming) while continuing to deliver net energy for the needs of a growing global society.

Professor Jeroen van den Bergh

A dynamic EROI model has been developed by van den Bergh and King to inspect the net energy delivered to society, considering investment as well as operational expenses. The authors also suggest an 'energy return on carbon' (EROC) indicator - a metric of net energy per tCO2 - to help maximize the potential net energy from the 2 ºC carbon budget. This would make it possible to compare the performance of various energy sources under the limitation of a climate change target.

As per the EROC indicator, natural gas with carbon capture and storage (CCS) comes out best. It is significantly better than coal with CCS and performs more than 10 times better than tar sands and oil shale. On the other hand, among fossil fuels tar sands and oil shale are extremely bad choices in terms of climate risks.

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